The present invention is related to assemblies for tunable lasers and, in particular, to assemblies for finely tuning a resonance cavity for a laser.
Tunable lasers, i.e., lasers having output wavelengths which can be varied in a controlled manner, have many applications. For example, one such application is for a fiber optic network in which tunable lasers may be used with wavelength dependent multiplexers to direct optical signals to particular points in the network.
Thus, it is very desirable that the laser be tuned accurately. One way of tuning a laser is through the use of a resonance cavity formed by the laser at one end and a diffraction grating at the other end. By carefully rotating the grating, standing waves of a particular wavelength are created in the cavity. In other words, the frequency of the light emitted by the laser is tuned by rotating the grating to select the desired frequency.
Previous ways of rotating the grating have included the use of a piezo-electric ceramic or of a stepper motor. In the latter case, the grating is mounted to the shaft of a stepper motor, which typically has thousands of steps per revolution. This does not provide sufficient resolution, i.e., no better than 1 GHz (due to mode hopping and ppor side-mode suppression) over a wide tuning range. A silica plate between the end of the laser and the refraction grating is used for the fine tuning of the frequency, up to 1 Mhz. Other approaches have included the use of expensive microstepper motors with and without shaft encoders to achieve sufficient discrimination.